twisted/test/stdio_test_halfclose.py typo

Description

So, I had brought this up on IRC awhile back, but it appears that

source:/trunk/twisted/test/stdio_test_halfclose.py@current#L20

has the string "???" inserted as a placeholder.

This is especially an issue for Debian, as every time you install a Python-related package it recompiles the site-packages directory, and if that fails, it tends to stop the package install/config process.